The present invention relates to an improved acoustic wave generating device, adapted for lowering into a well of borehole at the end of a cable for generating acoustic waves by causing a falling mass to strike a target element coupled with the wall of the well.
The acoustic wave generating device of the invention finds its applications particularly in the field of seismic prospection.
A device is known for generating seismic impulses by causing a mass to strike a target element anchored in a borehole. The target element is fixed to an elongated body provided with anchorage means. The mass is movable inside the body between a bottom position in contact with the target element and a top position from where its fall is tripped by control means. The control means comprise an element with retractable hooks adapted for gripping an engagement piece fixed to the mass and rigid traction elements (a drill-pipe or interconnected drill-pipe string) connected to lifting means disposed inside the well. The anchorage means are of the well known packer type and comprise an expandable member formed of a radially movable peripheral portion and a central portion. Expansion is obtained by causing the two portions to rotate with respect to each other by rotating the drill pipe string from the surface. The peripheral portion may be formed of pivoting arms provided with claws which, by opening, anchor themselves in the wall of the well, or else a deformable enclosure.
Such a device is described in the published French patent application 2 590 994. From another published European patent application EP 242262, and corresponding U.S. Pat. No. 4,770,268 another device is known for generating seismic waves in a borehole by causing a mass falling inside an elongated body to strike an anchored target element which is provided with a resetting system which raises the mass from underneath and moves the mass by translation to a tripping position.
This system comprises a threaded rod disposed in the longitudinal axis of the body and a nut locked against rotation which is threaded thereon. The nut is disposed under the mass, and rotation of the rod in a certain direction moves the nut which drives the mass to a top position where the nut is locked in a notch of an engagement bar disposed in the body along a generatrix thereof. By then changing the direction of rotation of the threaded rod, the nut is brought back to a bottom position in which it is moved laterally away from the engagement bar and frees the mass. The threaded rod is driven from the surface by a rigid drill-pipe string or else by an electric motor disposed above the body and fed from the surface by an electric supply and support cable.
The use of a rigid drill-pipe string makes it possible to apply a considerable tractive force and so to reset a well source up to very great depths even in highly deflected wells. But because of the rigid connection existing permanently with the surface, a part of the energy of the shocks produced is transmitted along the well and gives rise to parasitic emissions which disturb somewhat the reception of the useful echos corresponding to the waves transmitted directly to the walls at the anchorage depth of the well source.
Resetting a seismic source using a weight dropping technique by electric motors fed from the surface by an electric supply and support cable such as described in aforementioned European Patent Application 242262; overcomes the drawbacks of a rigid connection. The electric supply and support cable may be slackened before tripping the source, and the energy of the shocks is not transmitted thereby along the well. But the relatively low electric power which the cables generally used in wells may transmit limits the capabilities of such a source. When the dropping height required to obtain powerful impacts is considerable, the time require for resetting is extended, which reduces the possible rate of seismic "shots". If the resetting time intervals are imposed, the dropping height and so the power of the seismic source must necessarily be reduced.